In order to create a new invoice on an athlete's account, you will want to go to People > Athletes > search the athlete in question and click into their account.
Once in their profile, click Memberships & Payments and then scroll to the Invoice section
From here, click the (+) New Invoice Button:
You will then be directed to change the Payment Due date and the Location of Sale:
Make sure to click Save!
The new invoice will then be generated and you can start by adding a line item by clicking the Actions drop down:
You can also create an invoice by going to Financial > Invoices and clicking the (+) New Invoice button in blue and following the above steps!